1. Field of the Invention
The present invention relates to a machine tool having a workpiece table which has a clamping surface for workpieces to be machined; a spindle head displaceable in a working space above the workpiece table relative thereto by means of a shifting mechanism and having a spindle receiving tools for machining the workpieces; and three carriages movable orthogonally to one another in the shifting mechanism, of which the second carriage is mounted displaceably on the first, and the third carriage displaceably on the second carriage, the spindle head being joined to the third carriage.
2. Related Prior Art
A machine tool of this kind is known from the related art.
In the case of the known machine tool, the spindle head is displaceable in three mutually orthogonal coordinate directions, specifically the X, Y, and Z directions, in a working space above the workpiece table.
The shifting mechanism provided for this purpose comprises as the first carriage an X carriage which is arranged in the region of the clamping surface of the workpiece table, preferably below the clamping surface. Arranged on this X carriage as the second carriage is the Y carriage, which is displaceable perpendicular to the X carriage so that the X and Y carriages enclose a plane which lies parallel to the clamping plane.
Arranged on the Y carriage is an upright which extends over the entire height of the working space and bears at its upper end the third carriage, i.e. the Z carriage, to which the spindle head is attached and thus projects from above into the working space. In other words, the upright extends on the rear wall (as viewed from the operator side) of the working space.
Depending on the height of the working space, the upright has a relatively large mass, so that kinematic limits are imposed on the maximum acceleration achievable upon displacement of the spindle head in the X or Y direction. Acceleration and deceleration of the upright cannot be attained in arbitrarily short time spans.
The known machine tool further has an enclosure which not only covers the machine tool externally but also provides a lining for the working space, so as to protect the shifting mechanism from chips as well as cutting and cooling oil. The carriages and guide rails as well as the upright are protected, in this context, by telescoping covers which constitute what may be called variable-length covers, so that the parts of the machine tool projecting from the shifting mechanism into the working space do not need to pass through slits in the enclosure, through which chips and cooling or cutting fluid might get into the shifting mechanism.
The telescoping covers comprise U-shaped sheet-metal elements which can slide over one another and are, for example, arranged on both sides of the X carriage and overlap the guides for said carriage and thus protect them from soiling.
All in all, large surfaces must be sealed in the case of the known machine tool, and the variable-length telescoping covers are provided for them.
One disadvantage with the telescoping covers is their large mass, which results not only from their design but also from the large surfaces to be sealed, so that the telescoping covers also contribute to the acceleration problems already mentioned above. A further disadvantage is that the sheet-metal parts strike one another when being slid together or moved apart, especially at high displacement velocities. These impacts not only represent a noise problem, but also impair the machining accuracy of the known machine tool due to the vibrations associated with them.